The present invention relates to an inspection apparatus using nuclear magnetic resonance for acquiring echoes while monitoring motion of a subject using a nuclear magnetic resonance signal.
A nuclear magnetic resonance imaging (MRI) apparatus is a medical image diagnosis apparatus which generates nuclear magnetic resonance in a hydrogen atomic nucleus in an arbitrary plane across a subject to acquire a tomographic image in the plane from a nuclear magnetic resonance signal produced.
In general, a slice select magnetic field gradient specifying a plane to acquire a tomographic image of a subject is applied, and at the same time, an excitation pulse exciting spins in the plane is given, so as to acquire a nuclear magnetic resonance signal (echo) produced in the stage that the spins excited is refocused. To give position information to the spins, a phase-encoding magnetic field gradient and a readout magnetic field gradient in the direction vertical to each other in a transverse section are applied during the period between the excitation and the echo acquisition. The measured echo is arranged in a k-space in which the horizontal axis is kx and the vertical axis is ky. One echo occupies one line parallel to the kx axis. The k-space is inverse Fourier transformed to perform image reconstruction.
The pulse and the magnetic field gradient for producing an echo are applied based on a predetermined pulse sequence. Various types of the pulse sequence are known according to an object.
For example, a gradient-echo (GE) method, one of general echo-acquiring methods, is a method which repeatedly operates its pulse sequence to sequentially change a phase-encoding magnetic field gradient for each repetition, thereby successively measuring the number of echoes necessary for acquiring one tomographic image.
FIG. 1(A) shows a pulse sequence of the GE method. The operation of the pulse sequence is as follows. A slice select magnetic field gradient pulse 201 in z direction is applied. At the same time, a radiofrequency magnetic field (RF) pulse 202 for spin excitation having resonance frequency f0 of a proton is applied. A nuclear magnetic resonance phenomenon is induced in the proton of a certain slice in a subject. A phase-encoding magnetic field gradient pulse 203 for adding position information in a phase-encoding direction (y) to the phase of spin and a readout magnetic fields gradient 205 for dephase are applied. While applying a readout magnetic field gradient pulse 206 for adding position information in a readout direction (x), a nuclear magnetic resonance signal (echo) 208 is measured. After the echo measurement, a re-phase magnetic field gradient pulse 207 is applied to return the phase of the spin for the next excitation.
The procedure from the slice select magnetic field gradient pulse application to the echo measurement is repeated in repetition time TR to measure echoes necessary for acquiring one image. The echoes are arranged on a k-space 209, as shown in FIG. 1(B), for image reconstruction by two-dimensional inverse Fourier transform. The echo-acquiring time per image is 1.28 seconds when at TR of 10 ms, an image of 128×128 pixels is acquired. The TR of the sequence is short. The sequence must be executed above about 20 times until the spin is in a steady state so that the echoes are stable.
In the case of cardiac imaging, since a cardiac cycle is about one second which is shorter than the imaging time, ECG gating is widely used to improve the time resolution (for example, see “NMR Igaku-Kiso to Rinsho”, published by Maruzen, 1991, pp. 144 to 145). This method is a method which changes phase encoding in synchronization with a trigger of R wave of an ECG to measure echoes necessary for reconstruction of one image across plural heartbeats. When respiratory motion occurs during echo-acquiring, a ghost is caused in the reconstructed image. Echo-acquiring is generally done during breath-hold.
As an example thereof, FIG. 2(A) shows the relation between an ECG and measurement when an image of 128×128 pixels is acquired with 8 frames per heartbeat and TR of 5 ms. First, phase encoding is changed by 8 immediately after R wave T1 to perform measurement from −64 to 56. This is repeated for 8 frames 8 times. Then, the phase encoding is also changed by 8 immediately after R wave T2 to perform measurement from −63 to 57. This is repeated for 8 frames 8 times. The above measurement is continuously performed to T8. As shown in FIG. 2(B), the echo 208 is rearranged for each of the frames in order of phase encoding to be arranged on the k-space 209 for reconstruction.
There is proposed a method for monitoring breathing using a nuclear magnetic resonance signal. This is a method for detecting motion of the boundary (diaphragm) between a liver and a lung from a one-dimensional projection in an area excited in a rod form from the liver to the lung (see U.S. Pat. No. 5,363,844).
In the prior art method for using ECG gating to enhance the time resolution of echo-acquiring, the echo-acquiring time of one image is about 8 seconds for 8 heartbeats. The breath-holding time of a health person is up to about 30 seconds. The number of slices which can be echo-acquired with one breath-hold is about 4 at most. When echo-acquiring the number of slices larger than that, breath-hold must be done plural times. This puts a significant load on a subject.
In the prior art method for monitoring breathing using a nuclear magnetic resonance signal, an echo-acquired section and an excitation area for breathing monitoring are different. Each time breathing monitoring is executed, an extra sequence for returning spin to a steady state must be executed. The echo-acquiring time is longer.
In the prior art, a heart is echo-acquired by ECG gating. An ECG must be worn on a subject.